Fallback to copy symlinks on Windows

Symlinks are not permitted for an ordinary Windows user.
To use them, a switch called "Development Mode" in the system settings has to be enabled.

This prevents users per default to install gems using symlinks.
One such example is haml-rails-3.0.0.
It uses symlinks for files and directories.
The resulting error message is not very helpful:

```
ERROR:  While executing gem ... (Gem::FilePermissionError)
    You don't have write permissions for the directory. (Gem::FilePermissionError)
```

Instead of fixing the situaltion in the affected gem or to skip symlinks completely,
I think the better solution would be to make copies of the files in question.
This would allow Windows users to install and use the gem smoothly.

Author: larskanis

lib/rubygems/package.rb

@@ -466,7 +466,7 @@ def extract_tar_gz(io, destination_dir, pattern = "*") # :nodoc:
 
     symlinks.each do |name, target, destination, real_destination|
       if File.exist?(real_destination)
-        File.symlink(target, destination)
+        create_symlink_safe(target, destination)
       else
         alert_warning "#{@spec.full_name} ships with a dangling symlink named #{name} pointing to missing #{target} file. Ignoring"
       end
@@ -721,6 +721,16 @@ def limit_read(io, name, limit)
     raise Gem::Package::FormatError, "#{name} is too big (over #{limit} bytes)" if bytes.size > limit
     bytes
   end
+
+  # Create a symlink and fallback to copy the file or directory on Windows,
+  # where symlink creation needs special privileges in form of the Developer Mode.
+  def create_symlink_safe(old_name, new_name) # :nodoc:
+    File.symlink(old_name, new_name)
+  rescue Errno::EACCES
+    raise unless Gem.win_platform?
+    from = File.expand_path(old_name, File.dirname(new_name))
+    FileUtils.cp_r(from, new_name)
+  end
 end
 
 require_relative "package/digest_io"